IPv4: Basic implementation of TCP socket shutdown

We can now participate in the TCP connection closing handshake. :^)
This implementation is definitely not complete and needs to handle a
bunch of other cases. But it's a huge improvement over not being able
to close connections at all.

Note that we hold on to pending-close sockets indefinitely, until they
are moved into the Closed state. This should also have a timeout but
that's still a FIXME. :^)

Fixes #428.

Kernel/Net/IPv4Socket.cpp

@@ -500,3 +500,8 @@ int IPv4Socket::ioctl(FileDescription&, unsigned request, unsigned arg)
 
     return -EINVAL;
 }
+
+void IPv4Socket::close()
+{
+    shutdown(SHUT_RDWR);
+}